add22f5957 added error checks to all the
passToPacman commands. This makes `yay -Q nonexistantpackage` return
non 0 as it should. Annoyingly it also made yay print `exit status = n`
which is the error string from passToPacman calls. This error doesn't
add much and is quite annoying, expecially when calling pacman commands
like `-Q` or `-Si` where yay should be kind of 'hidden' and print just
like pacman does.
Now set the error string to "" for pacman commands and don't print an
error if it == "" (avoids empty line printed).
Also behave more like pacman when using `yay -Qu`.
